1. Field of the Invention
This invention relates to the surface treating method of single crystals, such as the single-crystal substrates, etc., used for forming superconducting oxide thin films.
2. Statement of the Related Art
A crystal has a three-dimensional internal structure along which atoms are systematically arranged. Bond strengths act between the atoms and molecules constituting the crystal and, because of the bond strengths, the atoms and molecules are systematically arranged in a lattice, resulting in the crystal. Each crystal has its own crystal structure which varies depending upon the crystal system.
Oxide single crystals can be classified into oxide single crystals for optical use, single crystals for substrates used for superconducting thin films, etc., according to their application.
The oxide single crystals for optical use start to attract attention as optical functional elements, the optical communication technology is being developed and researches into the optical computer are in progress.
The single crystals for substrates used for forming superconducting thin films are used in the field of electronics. In the field of electronics, elements utilizing superconductivity are used as Josephon elements represented by the SQUID (high-sensitivity magnetic flux detecting element), superconducting transistors, optical and electromagnetic sensors, etc., and most of the elements are being studied so as to mount them on substrates in the form of thin films. As the research into these elements progress, demands for better substrates are created and, therefore, novel substrates are sought and better treating methods are studied. Recently, many authors reported their research and developed results of single-crystal substrates for thin film elements which are considered to be most quickly put to practical use. For example, bi-crystal substrates have been developed for the SQUID. Single crystals for substrates used for superconducting thin films include LaAlO.sub.3 (lanthanum aluminate), SrTiO.sub.3, NdAlO.sub.3 (neodymium aluminate), LaGaO.sub.3 (lanthanum gallate), NdGaO.sub.3 (neodymium gallate), YAlO.sub.3 (yttrium aluminate), Nb:SrTiO.sub.3 (niobium-doped strontium titanate), etc.
The requirements of single crystals for substrates for superconducting thin films are described below:
1. Their lattice constants must be close to those of thin films (crystal structures close to those of thin films are also desirable). PA1 2. Their coefficients of thermal expansion must be close to those of thin films. PA1 3. They should not react to thin films (their constituent elements should not be diffused into thin films). PA1 4. They must not cause phase transition within the working temperature ranges of thin films (from 4.2K to film forming temperatures). PA1 5. They must have small dielectric constants (small dielectric loss tangents (tan .delta.) are also desirable). PA1 6. High-quality crystals must be obtained. PA1 7. Large-sized substrates must be formed from them. PA1 8. They must be supplied stably (desirably, at low costs). PA1 9. They must be free from surface distortion (deformed layers resulting from treatment). PA1 10. They must have flat surfaces.
Of the above-mentioned requirements, requirements 1 to 5, 6 to 8, and 9 and 10 respectively relate to the characteristics, growing method, and working method of single crystals for substrates.
When the generally used surface treating method using hard abrasive grains, such as those of diamond, etc., is used for the surface finish of single crystals for substrates, the distortion (deformation) of crystal surfaces reaches several to several tens .mu.m and such crystals cannot be used for superconducting thin films. Therefore, so-called mechanochemical polishing using soft abrasive grains, such as colloidal silica particles, etc., or chemical polishing which uses chemicals is used for the surface finish or sputter-etching is performed on the surfaces before film formation.
At the time of manufacturing artificial oxide lattices or developing multi-layered devices with the purpose of seeking novel superconducting materials and devices, the surface conditions of the single crystals used for substrates become very important. SrTiO.sub.3 single crystals polished by mechanochemical polishing with colloidal silica particles are widely used for forming substrates for superconducting thin films. However, their surfaces have a surface roughness of 3-40 .ANG. in maximum height (P-V) and such roughness is not sufficiently flat to deposit high-quality superconducting thin films.
The purpose of this invention is to provide a surface treating method by which the surfaces of single crystals can be finished to surfaces having small surface roughness or flat surfaces.
The second purpose of this invention is to provide surface treatment by which single crystals having pit-free step structures can be obtained even when surface treating conditions are relaxed.
The third purpose of this invention is to obtain better step structures even under such a treating condition that no pit occurs or pits hardly occur.